1. Field of the Invention
The present invention relates to an induction-type position measuring apparatus for performing position measurements based on electromagnetic coupling principles.
2. Description of the Related Art
One prior known magnetic encoder employing transformers is configured including a displacement sensor and a scale structure operatively associated therewith. The displacement sensor typically has two coils: a primary coil (i.e. drive coil), and a secondary coil (detection coil). The scale is placed adjacent to the sensor and is movable relative thereto. The scale is responsible during movement for modulating the magnetic flux as generated from the drive coil. Upon activation of the drive coil by supplying an alternating drive current thereto, variable magnetic flux might take place, which is then modulated by the scale resuting in coupling with the detection coil. This magnetic coupling causes induction of a corresponding voltage at the detector coil. The induced voltage is variable in potential with movement of the scale, and thus is employable as a detection output indicative of a position measured.
With the prior art magnetic encoder thus arranged, the drive coil and its associative detector coil are coaxially disposed. Such coaxial coil layout must permit presence of electromagnetic coupling components (cross-talk) between the spaced-apart drive coil and detector coil, which components remain independent of the relative position of the scale and are hardly affected from it. Presence of such crosstalk results in inclusion of certain offset components in a detection output of the detector coil, which again are hardly affectable by relative movements of the scale. These offset components can badly behave to cause significant measurement errors in cases where high-precision position detection is performed by dividing a wavelength of the scale into portions.
One typical approach to removal of such offset components is to differentiate two detection voltages of which phases are reverse with each other. This approach, however, suffers from difficulty in offset removal due to the fact that offset components are significantly affectable from the alignment accuracy of the drive and detector coils along with uniformity of materials employed therefor, resulting in the offset value experiencing non-negligible amount of variations among individual products.